Exhaust gas purification system for an internal combustion engine

ABSTRACT

The present invention is to provide a technique which can detect the level of degradation of a precatalyst in an exhaust gas purification system for an internal combustion engine that includes a NOx catalyst arranged on an exhaust passage of the internal combustion engine, and the precatalyst arranged on the exhaust passage at a location upstream of the NOx catalyst and having an oxidation function. In the present invention, the level of degradation of the precatalyst is detected based on the width of change in temperature of the NOx catalyst at the time when a reducing agent is intermittently supplied from an upstream side of the precatalyst to the precatalyst and the NOx catalyst. At this time, it is determined that the larger the width of change in temperature of the NOx catalyst, the higher the level of degradation of the oxidation catalyst is.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification system foran internal combustion engine which includes an NOx storage-reductioncatalyst arranged on an exhaust passage of the internal combustionengine, and a precatalyst that is arranged on an exhaust passage at alocation upstream of the NOx storage-reduction catalyst and has anoxidation function.

PRIOR ART

In Japanese patent application laid-open No. 2000-310113, there isdisclosed an exhaust gas purification system for an internal combustionengine which includes an exhaust gas purification catalyst that isarranged on an exhaust passage of the internal combustion engine, and anadsorption means that is arranged on the exhaust passage at a locationdownstream of the exhaust gas purification catalyst and adsorbsprescribed components of an exhaust gas, wherein the degradation of theexhaust gas purification catalyst is determined based on an amount ofthe prescribed components adsorbed by the adsorption means.

Also, in Japanese patent application laid-open No. 2001-342879, there isdisclosed an exhaust gas purification system for an internal combustionengine which includes an NOx storage-reduction catalyst (hereinaftersimply referred to as a NOx catalyst) that is arranged on an exhaustpassage of the internal combustion engine, and a precatalyst that isarranged on the exhaust passage at a location upstream of the NOxcatalyst and has an oxidation function, wherein when the NOx occluded inthe NOx catalyst is reduced, the air fuel ratio of an exhaust gas iscontrolled in accordance with the level of degradation of theprecatalyst.

DISCLOSURE OF THE INVENTION

The present invention is intended to provide a technique which candetect the level of degradation of a precatalyst in an exhaust gaspurification system for an internal combustion engine that includes aNOx catalyst arranged on an exhaust passage of the internal combustionengine, and the precatalyst arranged on the exhaust passage at alocation upstream of the NOx catalyst and having an oxidation function.

The present invention detects the level of degradation of theprecatalyst based on the width of change in temperature of the NOxcatalyst at the time when a reducing agent is intermittently suppliedfrom an upstream side of the precatalyst to the precatalyst and the NOxcatalyst.

More specifically, an exhaust gas purification system for an internalcombustion engine according to the present invention is characterized bycomprising:

an NOx storage-reduction catalyst that is arranged on an exhaust passageof the internal combustion engine,

a precatalyst that is arranged on an exhaust passage at a locationupstream of said NOx storage-reduction catalyst and has an oxidationfunction,

a temperature detecting means that detects the temperature of said NOxstorage-reduction catalyst,

a reducing agent supplying means that intermittently supplies a reducingagent to said precatalyst and said NOx storage-reduction catalyst froman upstream side of said precatalyst, and

a degradation level detection means that detects the level ofdegradation of said precatalyst based on the width of change intemperature of said NOx storage-reduction catalyst at the time when saidreducing agent is intermittently supplied by said reducing agentsupplying means.

When the level of degradation of the precatalyst is relatively low, theoxidation of the reducing agent supplied from the reducing agentsupplying means is liable to be facilitated in the precatalyst. When thereducing agent is oxidized in said precatalyst, the temperature of theprecatalyst rises due to the oxidation heat, and at the same time thetemperature of the NOx catalyst rises, too. In case where the oxidationof the reducing agent in the precatalyst is liable to be facilitated,the amount of the reducing agent and the amount of oxygen in the exhaustgas which reach up to the NOx catalyst become relatively small.Therefore, the amount of the reducing agent oxidized in the NOx catalystnecessarily decreases. As a result, the amount of oxidation heatgenerated due to the oxidation of the reducing agent in the NOx catalystdecreases. Accordingly, in case where the level of degradation of theprecatalyst is relatively low, the temperature itself of the NOxcatalyst rises higher when the intermittent supply of the reducing agentis executed by the reducing agent supplying means as compared with thetime when the supply of said reducing agent is not executed, but thewidth of change in temperature of the NOx catalyst during the time whenthe reducing agent is intermittently supplied becomes small.

On the other hand, when the level of degradation of the precatalyst isrelatively high, the reducing agent supplied from the reducing agentsupplying means is difficult to be oxidized in the precatalyst. Thus,the temperature of the precatalyst is also difficult to rise. In such acase, the amount of the reducing agent and the amount of oxygen in theexhaust gas which reach up to the NOx catalyst become relatively large.As a result, the amount of oxidation heat generated due to the oxidationof the reducing agent in the NOx catalyst increases. Accordingly, adifference in temperature of the NOx catalyst between the times when thereducing agent is supplied to the NOx catalyst and when it is notsupplied becomes large. In other words, when the level of degradation ofthe precatalyst is relatively high, the width of change in temperatureof the NOx catalyst during the time when the reducing agent isintermittently supplied from the reducing agent supplying means becomeslarge.

In this manner, the width of change in temperature of the NOx catalystat the time when the reducing agent is intermittently supplied by thereducing agent supplying means varies according to the level ofdegradation of the precatalyst. Accordingly, the level of degradation ofthe precatalyst can be detected based on the width of change intemperature of the NOx catalyst at this time.

The reducing agent becomes less liable to be oxidized in the precatalystin accordance with the increasing level of degradation of theprecatalyst. Thus, the temperature of the precatalyst becomes difficultto rise, and the amount of the reducing agent and the amount of oxygenin the exhaust gas, which reach the NOx catalyst when the reducing agentis supplied by the reducing agent supplying means, increase.Accordingly, when the reducing agent is intermittently supplied by thereducing agent supplying means, the temperature of the NOx catalyst atthe time when the reducing agent is not supplied becomes lower and thetemperature of the NOx catalyst when the reducing agent is suppliedbecomes higher, in accordance with the increasing level of degradationof the precatalyst.

Accordingly, in the present invention, it can be determined that thelarger the width of change in temperature of the NOx catalyst at thetime when the reducing agent is intermittently supplied by the reducingagent supplying means, the higher the level of degradation of theprecatalyst is.

In the present invention, a NOx reduction control execution means thatexecutes NOx reduction control for reducing the NOx occluded in the NOxcatalyst by intermittently supplying the reducing agent by means of thereducing agent supplying means may further be provided.

As stated above, the higher the level of degradation of the precatalyst,the more the amount of the reducing agent and the amount of oxygen inthe exhaust gas become, which can reach the NOx catalyst when thereducing agent is supplied by the reducing agent supplying means. Inthis case, there is fear that even if the reducing agent is supplied soas to reduce the NOx occluded in the NOx catalyst, the efficiency of NOxreduction control might decrease because of the oxidation reaction ofthe reducing agent also occurring with the reduction reaction of NOx inthe NOx catalyst.

Accordingly, in the above case, a precatalyst temperature raising meansthat serves to make the temperature of the precatalyst higher, in casewhere the width of change in temperature of the NOx catalyst at the timewhen the reducing agent is intermittently supplied by the reducing agentsupplying means (hereinafter simply referred to as the temperaturechange width of the NOx catalyst) is equal to or larger than apredetermined change width when the NOx reduction control is executed bythe NOx reduction control execution means, than in case where thetemperature change width of the NOx catalyst is less than thepredetermined change width may further be provided.

Here, the predetermined change width is a threshold value with which itcan be determined that the degradation of the precatalyst has progressedto such an extent that the efficiency of NOx reduction control islowered excessively.

The oxidation of the reducing agent in the precatalyst can befacilitated by raising the temperature of the precatalyst. That is, theamount of the reducing agent and the amount of oxygen in the exhaustgas, which reach up to the NOx catalyst, can be decreased.

Thus, according to the above, the efficiency decrease of the NOxreduction control can be suppressed even in a state where thedegradation of the precatalyst has progressed.

In addition, in the above case, the larger the temperature change widthof the NOx catalyst, the higher a target temperature at the time whenthe temperature of the precatalyst is raised by the precatalysttemperature raising means may be made.

According to this, even if the level of degradation of the precatalystbecomes much higher, it is possible to facilitate the oxidation of thereducing agent in the precatalyst.

In the above case, the target temperature is set in accordance with thetemperature change width of the NOx catalyst, i.e., in accordance withthe level of degradation of the precatalyst. However, when thedegradation of the precatalyst has progressed to an excessive extent, itbecomes difficult to oxidize the reducing agent in the precatalyst to asatisfactory extent even if the temperature of the precatalyst is causedto rise.

Accordingly, in case where the temperature of the precatalyst is causedto rise, as stated above, a predetermined upper limit value may be setfor the target temperature, so that the NOx reduction control of the NOxreduction control execution means can be inhibited from being executedwhen the target temperature set in accordance with the temperaturechange width of the NOx catalyst becomes higher than the predeterminedupper limit value.

Here, note that the predetermined upper limit value is a temperatureequal to or lower than a threshold with which it can be determined thatthe degradation of the precatalyst has progressed to such an extent thatit is difficult to oxidize the reducing agent in the precatalyst to asatisfactory extent even if the temperature of the precatalyst is causedto rise, when the target temperature is set to a value higher than thepredetermined upper limit value.

According to the above, it is possible to suppress the NOx reductioncontrol from being executed in a state where it is difficult to reducethe NOx occluded in the NOx catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the schematic construction of intake andexhaust systems of an internal combustion engine according to anembodiment of the present invention.

FIG. 2 is a view showing the temperature change of a NOx catalyst whenintermittent addition of fuel from a fuel addition valve is executed.

FIG. 3 is a flow chart illustrating a routine for NOx reduction controlaccording to the embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

Hereinafter, a specific embodiment of an exhaust gas purification systemfor an internal combustion engine according to the present inventionwill be described while referring to the drawings.

<Schematic Construction of Intake and Exhaust Systems of an InternalCombustion Engine>

Here, reference will be made, by way of example, to a case where thepresent invention is applied to a diesel engine used for driving avehicle. FIG. 1 is a view that shows the schematic construction ofintake and exhaust systems of an internal combustion engine according toan embodiment of the present invention.

The internal combustion engine 1 is the diesel engine for driving thevehicle. An intake passage 3 and an exhaust passage 2 are connected withthis internal combustion engine 1. A throttle valve 7 is arranged on theintake passage 3. An oxidation catalyst 4 and a NOx catalyst 5 arearranged on the exhaust passage 2.

The NOx catalyst 5 is a catalyst that serves to occlude NOx in anexhaust gas when the surrounding atmosphere is an oxidative atmosphere,and to reduce the NOx thus occluded when the surrounding atmosphere is areductive atmosphere. The NOx catalyst 5 is arranged on the exhaustpassage 2 at a location downstream of the oxidation catalyst 4. Here,note that in this embodiment, the oxidation catalyst 4 corresponds to aprecatalyst according to the present invention. The precatalyst needsonly to be a catalyst having an oxidation function, and for example, theoxidation catalyst 4 may be a NOx catalyst, and the NOx catalyst 5 maybe a particulate filter having the NOx catalyst carried thereon.

A fuel addition valve 6 for adding fuel as a reducing agent to theexhaust gas is arranged on the exhaust passage 2 at an upstream side ofthe oxidation catalyst 4. In the present invention, the fuel additionvalve 6 corresponds to a reducing agent supplying means.

Further, a first temperature sensor 8 and a second temperature sensor 9,which detect the temperature of the exhaust gas are arranged on theexhaust passage 2 at a location between the oxidation catalyst 4 and theNOx catalyst 5 and at a location downstream of the NOx catalyst 5,respectively.

An electronic control means (ECU) 10 for controlling the internalcombustion engine 1 is provided in conjunction with the internalcombustion engine 1 as constructed in the above-described manner. Thefirst temperature sensor 8 and the second temperature sensor 9 areelectrically connected to the ECU 10, and output signals of thesesensors are input to the ECU 10. The ECU 10 estimates the temperature ofthe oxidation catalyst 4 based on the output value of the firsttemperature sensor 8, and estimates the temperature of the NOx catalyst5 based on the output value of the second temperature sensor 9. In thisembodiment, the second temperature sensor 9 corresponds to a temperaturedetection means according to the present invention.

In addition, the throttle valve 7 and the fuel addition valve 6 areelectrically connected to the ECU 10, so that these valves arecontrolled by the ECU 10.

<NOx Reduction Control>

In this embodiment, NOx reduction control is performed so as to reducethe NOx occluded in the NOx catalyst 5. The NOx reduction controlaccording to this embodiment is executed by intermittently adding fuelfrom the fuel addition valve 6 at the time when the temperature of theNOx catalyst 5 is an activation temperature. Fuel is supplied to the NOxcatalyst 5 by the fuel being added from the fuel addition valve 6. As aresult, the air fuel ratio in the surrounding atmosphere of the NOxcatalyst 5 decreases, whereby the surrounding atmosphere becomes areduction atmosphere, so the NOx occluded in the NOx catalyst 5 isreduced. In addition, the excessive temperature rise of the oxidationcatalyst 4 and the NOx catalyst 5 can be suppressed by intermittentlyperforming the addition of fuel from the fuel addition valve 6.

<Degradation Level Detection Method for Oxidation Catalyst>

The NOx reduction control is performed by intermittently adding fuelfrom the fuel addition valve 6, as stated above. At this time, the fueladded from the fuel addition valve 6 is supplied to the oxidationcatalyst 4 before it reaches the NOx catalyst 5.

The fuel supplied to the oxidation catalyst 4 is oxidized in theoxidation catalyst 4. The oxygen in the exhaust gas is consumed due tothe oxidation of fuel in the oxidation catalyst 4, so the amount ofoxygen in the exhaust gas, which reaches the NOx catalyst 5, isdecreased. Therefore, the air fuel ratio in the surrounding atmosphereof the NOx catalyst 5 becomes liable to decrease.

However, as the degradation of the oxidation catalyst 4 progresses, fuelbecomes less liable to be oxidized in the oxidation catalyst 4.Accordingly, when fuel is added from the fuel addition valve 6, theamount of the fuel that reaches up to the NOx catalyst 5 increases, andat the same time the amount of oxygen in the exhaust gas that reaches upto the NOx catalyst 5 increases, too. As a result, the reductionreaction of NOx and the oxidation reaction of fuel occur in the NOxcatalyst 5, thus giving rise to a fear that the efficiency of NOxreduction control might decrease.

Therefore, in this embodiment, the level of degradation of the oxidationcatalyst 4 is detected at the time of the execution of the NOx reductioncontrol. Here, reference will be made to a method for detecting thelevel of degradation of the oxidation catalyst 4 according to thisembodiment. FIG. 2 is a view that shows the temperature change of theNOx catalyst 5 at the time when intermittent addition of fuel from thefuel addition valve 6 is executed. In FIG. 2, the axis of ordinaterepresents the temperature Tc of the NOx catalyst 5, and the axis ofabscissa represents time t.

In addition, in FIG. 2, a curve L1 denotes the temperature change of theNOx catalyst 5 at normal times, i.e., at the time when the degradationof the oxidation catalyst 4 is relatively low, and a curve L2 denotesthe temperature change of the NOx catalyst 5 in a state where thedegradation of the oxidation catalyst 4 progresses to some extent, i.e.,at the time when the degradation of the oxidation catalyst 4 isrelatively high. Also, a straight line L3 denotes the temperature of theexhaust gas that flows through the exhaust passage 2 upstream of theoxidation catalyst 4.

When fuel is intermittently added from the fuel addition valve 6 withthe level of degradation of the oxidation catalyst 4 being relativelylow, the most of the added fuel is oxidized in the oxidation catalyst 4.When the fuel is oxidized in the oxidation catalyst 4, the temperatureof the oxidation catalyst 4 rises due to the heat of oxidation, and atthe same time the temperature of the NOx catalyst 5 rises, too. In casewhere the oxidation of the fuel in the oxidation catalyst 4 is liable tobe facilitated, the amount of fuel and the amount of oxygen in theexhaust gas, which reach up to the NOx catalyst 5, become relativelysmall. Therefore, the amount of the fuel oxidized in the NOx catalyst 5necessarily decreases. As a result, the amount of oxidation heatgenerated due to the oxidation of the fuel in the NOx catalyst 5decreases. Accordingly, in case where the intermittent addition of fuelfrom the fuel addition valve 6 is performed with the level ofdegradation of the oxidation catalyst 4 being relatively low, thetemperature of the NOx catalyst 5 itself rises higher as compared withthe case where the addition of fuel is not executed, as shown by L1 inFIG. 2, but the width of change in temperature ΔTc of the NOx catalyst 5in the course of the intermittent addition of fuel (hereinafter simplyreferred to as the temperature change width ΔTc of the NOx catalyst 5)becomes small.

On the other hand, when the level of degradation of the oxidationcatalyst 4 is relatively high, the fuel added from the fuel additionvalve 6 is difficult to be oxidized in the oxidation catalyst 4.Therefore, even if the intermittent addition of fuel from the fueladdition valve 6 is executed, the temperature of the oxidation catalyst4 is difficult to rise. Accordingly, the temperature rise of the NOxcatalyst 4 in accordance with the temperature rise of the oxidationcatalyst 4 becomes small, too. In such a case, the amount of the fueland the amount of oxygen in the exhaust gas, which reach up to the NOxcatalyst 5, are relatively large. As a result, the amount of oxidationheat generated due to the oxidation of the fuel in the NOx catalyst 5increases. Accordingly, a difference in temperature of the NOx catalyst5 between the times when fuel is supplied to the NOx catalyst 5 and whenit is not supplied becomes large. In other words, when the intermittentaddition of fuel from the fuel addition valve 6 is performed with thelevel of degradation of the oxidation catalyst 4 being relatively high,the temperature change width ΔTc of the NOx catalyst 5 becomes large, asshown by L2 in FIG. 2.

As can be seen from the foregoing description, the higher the level ofdegradation of the oxidation catalyst 4, the larger the width of changein temperature ΔTc of the NOx catalyst 5 at the time when fuel isintermittently added from the fuel addition valve 6 becomes.Accordingly, in this embodiment, the level of degradation of theoxidation catalyst 4 is detected based on the temperature change widthΔTc at the time of the execution of the NOx reduction control. Suchdetection of the level of degradation of the oxidation catalyst 4 iscarried out by the ECU 10. In this embodiment, the ECU 10, whichperforms the detection of the level of degradation of the oxidationcatalyst 4, corresponds to a degradation level detection means accordingto the present invention.

<Temperature Raising Control on the Oxidation Catalyst>

In addition, in this embodiment, when it can be determined that thedegradation of the oxidation catalyst 4 has progressed to some extent,the temperature of the oxidation catalyst 4 is controlled to rise so asto facilitate the oxidation of fuel in the oxidation catalyst 4 at thetime of the execution of the NOx reduction control.

Even in case where the degradation of the oxidation catalyst 4 hasprogressed to some extent, the oxidation of fuel in the oxidationcatalyst 4 can be facilitated by raising the temperature of theoxidation catalyst 4. As a result, the amount of fuel and the amount ofoxygen in the exhaust gas that reach up to the NOx catalyst 5 can bedecreased. Accordingly, a decrease in the efficiency of the NOxreduction control can be suppressed.

As the temperature raising control of the oxidation catalyst 4, therecan be exemplified the control of performing auxiliary fuel injection onthe expansion stroke in the internal combustion engine 1, the control ofdecreasing the degree of opening of the throttle valve 7, and so on.According to these, the temperature of the exhaust gas discharged fromthe internal combustion engine 1 is raised, in accordance with which thetemperature of the oxidation catalyst 4 is raised. In addition, in casewhere an EGR device is provided for introducing a part of the exhaustgas flowing through the exhaust passage 2 into the intake passage 3, theexhaust gas discharged from the internal combustion engine 1 may beraised in temperature by increasing the amount of the exhaust gasintroduced into the intake passage 3, as a result of which thetemperature of the oxidation catalyst 4 can be raised.

Also, as the temperature raising control of the oxidation catalyst 4,there can be exemplified the control of performing auxiliary fuelinjection on the exhaust stroke in the internal combustion engine 1. Inthis case, fuel, which is facilitated to be atomized by being injectedaccording to the auxiliary fuel injection, is supplied to the oxidationcatalyst 4, and the temperature of the oxidation catalyst 4 is raiseddue to the oxidation of the fuel being carried out by the oxidationcatalyst 4. In addition, by performing the addition of fuel from thefuel addition valve 6 in a more finely subdivided manner than inordinary or normal fuel addition, or by adding a fuel lighter than theone to be added in normal times, the oxidation of fuel in the oxidationcatalyst 4 can be facilitated, thereby making it possible to raise thetemperature of the oxidation catalyst 4.

Moreover, a heater may be arranged on the exhaust passage 2, so that theexhaust gas flowing into the oxidation catalyst 4 can be heated or theoxidation catalyst 4 itself can be raised in temperature by the heater.

<Routine for NOx Reduction Control>

Hereinafter, reference will be made to a routine for the NOx reductioncontrol according to this embodiment based on a flow chart shown in FIG.3. This routine is beforehand stored in the ECU 10, and is repeated atpredetermined time intervals.

In this routine, first in step S101, the ECU 10 determines whether anexecution condition for NOx reduction control holds. Here, as theexecution condition for NOx reduction control, there can be exemplifieda case in which an estimated value of an amount of NOx occlusion in theNOx catalyst 5 is equal to or more than a threshold for the execution ofNOx reduction control, and in which the temperatures of the oxidationcatalyst 4 and the NOx catalyst 5 are in the range of their activationtemperatures. When a positive determination is made in this S101, theECU 10 terminates this routine once, whereas when a negativedetermination is made, the ECU 10 proceeds to S102.

In S102, the ECU 10 executes the intermittent addition of fuel from thefuel addition valve 6. The amount of fuel to be added at this time iscontrolled in such a manner that the air fuel ratio of the exhaust gasflowing into the NOx catalyst 5 when the oxidation catalyst 4 is in anordinary or normal state, i.e., when the level of degradation of theoxidation catalyst 4 is relatively low, becomes equal to a target airfuel ratio. Here, note that the target air fuel ratio is an air fuelratio with which the surrounding atmosphere of the NOx catalyst 5becomes a reductive atmosphere. In addition, the stop interval of theaddition of fuel at this time is set so as to be able to suppress anexcessive rise in temperature of the oxidation catalyst 4 and the NOxcatalyst 5. In this embodiment, the ECU 10 executing this S102corresponds to the NOx reduction control execution means according tothe present invention.

Subsequently, the ECU 10 proceeds to S103, where a predetermined changewidth ΔT0, which is a temperature change width of the NOx catalyst 5 inthe form of a threshold for the degradation determination of theoxidation catalyst 4, is calculated. As stated above, the higher thelevel of degradation of the oxidation catalyst 4, the larger thetemperature change width of the NOx catalyst 5 becomes. In addition, thehigher the level of degradation of the oxidation catalyst 4, the lowerthe efficiency of the NOx reduction control becomes. Here, note that thepredetermined change width ΔT0 is a threshold value with which it can bedetermined that the degradation of the oxidation catalyst 4 hasprogressed to such an extent that the efficiency of NOx reductioncontrol might be lowered excessively. The predetermined change width ΔT0is calculated based on the amount of intake air, the amount of injectionfuel in the internal combustion engine 1, and the amount of fuel to beadded from the fuel addition valve 6.

Then, the ECU 10 proceeds to S104, where it determines whether thetemperature change width ΔTc of the NOx catalyst 5 is smaller than thepredetermined change width ΔT0 which is calculated in S103. When apositive determination is made in S104, the ECU 10 proceeds to stepS105, whereas when a negative determination is made, the ECU 10 proceedsto S106.

The ECU 10 having proceeded to S105 continues to execute theintermittent addition of fuel from the fuel addition valve 6. In otherwords, the execution of the NOx reduction control is continued.Thereafter, the ECU 10 once terminates the execution of this routine.

On the other hand, the ECU 10 having proceeded to S106 sets a targettemperature Toct in the temperature raising control of the oxidationcatalyst 4 based on the temperature change width ΔTc of the NOx catalyst5. Even when the degradation of the oxidation catalyst 4 progresses tosuch an extent that the temperature change width ΔTc of the NOx catalyst5 becomes the predetermined change width ΔT0 or more, the oxidation offuel in the oxidation catalyst 4 can be facilitated by raising thetemperature of the oxidation catalyst 4. That is, a decrease in theefficiency of the NOx reduction control can be suppressed. At this time,in order to facilitate the oxidation of fuel in the oxidation catalyst 4to a satisfactory extent, it is necessary to make the temperature of theoxidation catalyst 4 higher in accordance with the increasing level ofdegradation of the oxidation catalyst 4. Therefore, in this embodiment,the relation between the temperature change width ΔTc of the NOxcatalyst 5 and the target temperature Toct of the oxidation catalyst 4is beforehand determined in such a manner that the larger thetemperature change width ΔTc of the NOx catalyst 5, the higher thetarget temperature Toct of the oxidation catalyst 4 is set. That is, thehigher the level of degradation of the oxidation catalyst 4, the higherthe target temperature Toct is set.

Thereafter, the ECU 10 proceeds to S107, where it determines whether thetarget temperature Toct set in S106 is equal to or less than apredetermined upper limit value Toclimit. When the degradation of theoxidation catalyst 4 has progressed to an excessive extent, it becomesdifficult to oxidize the fuel in the oxidation catalyst 4 to asatisfactory extent even if the temperature of the oxidation catalyst 4is caused to rise. Here, note that the predetermined upper limittemperature Toclimit is a temperature in the form of a threshold withwhich it can be determined that the degradation of the oxidationcatalyst 4 has progressed to such an extent that it is difficult tooxidize the fuel in the oxidation catalyst 4 to a satisfactory extenteven if the temperature of the oxidation catalyst 4 is caused to rise,when the target temperature Toct is set to a value higher than thepredetermined upper limit temperature Toclimit. When a positivedetermination is made in S107, the ECU 10 proceeds to step S108, whereaswhen a negative determination is made, the ECU 10 proceeds to S109.

In S108, the ECU 10 executes the temperature raising control of theoxidation catalyst 4 according to the above-mentioned method, so thatthe temperature of the oxidation catalyst 4 is raised up to the targettemperature Toct. Thereafter, the ECU 10 proceeds to S105. In thisembodiment, the ECU 10 executing this S108 corresponds to a precatalysttemperature raising means according to the present invention.

On the other hand, in S109, the ECU 10 determines that it is difficultto reduce the NOx occluded in the NOx catalyst 5 to a satisfactoryextent in the NOx reduction control according to this embodiment, andstops the execution of the intermittent addition of fuel from the fueladdition valve 6. In other words, the execution of the NOx reductioncontrol is stopped. Thereafter, the ECU 10 once terminates the executionof this routine.

According to the routine as stated above, when the temperature changewidth ΔTc of the NOx catalyst 5 is equal to or larger than thepredetermined change width ΔT0, i.e., when it is determined that thedegradation of the oxidation catalyst 4 has progressed to such an extentthat the efficiency of NOx reduction control might be loweredexcessively, the temperature raising control of the oxidation catalyst 4is executed. Accordingly, the decrease in the efficiency of the NOxreduction control can be suppressed even in a state where thedegradation of the oxidation catalyst 4 has progressed.

In addition, in the temperature raising control of the oxidationcatalyst 4, the oxidation catalyst 4 is raised up to a highertemperature in accordance with the higher level of degradation of theoxidation catalyst 4. As a result, even if the level of degradation ofthe oxidation catalyst 4 becomes much higher, the oxidation of fuel inthe oxidation catalyst 4 can be facilitated.

Moreover, in case where the degradation of the oxidation catalyst 4progresses to such an extent that it is difficult to oxidize the fuel inthe oxidation catalyst 4 to a satisfactory extent even if thetemperature of the oxidation catalyst 4 is caused to rise, the executionof the NOx reduction control is stopped. Accordingly, it is possible tosuppress the NOx reduction control from being executed in a state whereit is difficult to reduce the NOx occluded in the NOx catalyst 5.

Here, note that in the above-mentioned routine, when the ECU 10 proceedsto S109, the execution of the NOx reduction control may be stopped, andat the same time, the driver of the vehicle, on which the internalcombustion engine 1 is installed, may be notified that the degradationof the oxidation catalyst 4 has progressed excessively, i.e., theoxidation catalyst 4 is in failure.

Further, in this embodiment, when the temperatures of the oxidationcatalyst 4 and the NOx catalyst 5 are in the range of activationtemperatures at a timing different from the timing at which NOxreduction control is executed, the intermittent addition of fuel fromthe fuel addition valve 6 may be executed, and the level of degradationof the oxidation catalyst 4 may be detected based on the temperaturechange width of the NOx catalyst 5 at this time.

In this case, the detected temperature change width of the NOx catalyst5 or the level of degradation of the oxidation catalyst 4 is stored inthe ECU 10, and at the time of the execution of the NOx reductioncontrol, it is determined, based on the value thus stored, whether thetemperature raising control of the oxidation catalyst 4 is to beexecuted. Further, a target temperature at the time when the temperatureraising control of the oxidation catalyst 4 is executed is set based onthe stored value.

In addition, when fuel is intermittently added from the fuel additionvalve 6 in control operations other than the NOx reduction control, thelevel of degradation of the oxidation catalyst 4 may be detected basedon the temperature change width of the NOx catalyst 5.

Moreover, in this embodiment, fuel in the form of the reducing agent issupplied to the oxidation catalyst 4 and the NOx catalyst 5 by addingthe fuel from the fuel addition valve 6 arranged on the exhaust passage2, but it may be possible to supply fuel to the oxidation catalyst 4 andthe NOx catalyst 5 by performing auxiliary fuel injection on the exhauststroke of the internal combustion engine 1, in place of the addition offuel from the fuel addition valve 6.

INDUSTRIAL APPLICABILITY

According to the present invention, in an exhaust gas purificationsystem for an internal combustion engine that includes a NOx catalystarranged on an exhaust passage of the internal combustion engine, and aprecatalyst arranged on the exhaust passage at a location upstream ofthe NOx catalyst and having an oxidation function, it is possible todetect the level of degradation of the precatalyst.

1. An exhaust gas purification system for an internal combustion engine,comprising: an NOx storage-reduction catalyst that is arranged on anexhaust passage of the internal combustion engine; a precatalyst that isarranged on the exhaust passage at a location upstream of said NOxstorage-reduction catalyst and has an oxidation function; a temperaturedetecting unit that detects the temperature of said NOxstorage-reduction catalyst; a reducing agent supplying unit thatintermittently supplies a reducing agent to said precatalyst and saidNOx storage-reduction catalyst from an upstream side of saidprecatalyst; and a degradation level detection unit that detects thelevel of degradation of said precatalyst based on the width of change intemperature of said NOx storage-reduction catalyst at the time when saidreducing agent is intermittently supplied by said reducing agentsupplying unit.
 2. The exhaust gas purification system for an internalcombustion engine as set forth in claim 1, wherein: said degradationlevel detection unit determines that the larger the width of change intemperature of said NOx storage-reduction catalyst at the time when thereducing agent is intermittently supplied by said reducing agentsupplying unit, the higher the level of degradation of said precatalystis.
 3. The exhaust gas purification system for an internal combustionengine as set forth in claim 1, comprising: an NOx reduction controlexecution unit that executes NOx reduction control for reducing the NOxoccluded in said NOx storage-reduction catalyst by intermittentlysupplying the reducing agent by unit of said reducing agent supplyingunit; and a precatalyst temperature raising unit that, when NOxreduction control is performed by said NOx reduction control executionunit, serves to make the temperature of said precatalyst higher, in casewhere the width of change in temperature of said NOx storage-reductioncatalyst at time when the reducing agent is intermittently supplied bysaid reducing agent supplying unit is equal to or more than apredetermined change width, than in case where the temperature changewidth of said NOx storage-reduction catalyst is less than saidpredetermined change width.
 4. The exhaust gas purification system foran internal combustion engine as set forth in claim 3, wherein: thelarger the width of change in temperature of said NOx storage-reductioncatalyst at the time when the reducing agent is intermittently suppliedby said reducing agent supplying unit, the higher a target temperatureat the time when the temperature of said precatalyst is raised by saidprecatalyst temperature raising unit is made.
 5. The exhaust gaspurification system for an internal combustion engine as set forth inclaim 4, wherein: the execution of the NOx reduction control by said NOxreduction control execution unit is inhibited when said targettemperature becomes higher than a predetermined upper limit value. 6.The exhaust gas purification system for an internal combustion engine asset forth in claim 2, comprising: an NOx reduction control executionunit that executes NOx reduction control for reducing the NOx occludedin said NOx storage-reduction catalyst by intermittently supplying thereducing agent by unit of said reducing agent supplying unit; and aprecatalyst temperature raising unit that, when NOx reduction control isperformed by said NOx reduction control execution unit, serves to makethe temperature of said precatalyst higher, in case where the width ofchange in temperature of said NOx storage-reduction catalyst at timewhen the reducing agent is intermittently supplied by said reducingagent supplying unit is equal to or more than a predetermined changewidth, than in case where the temperature change width of said NOxstorage-reduction catalyst is less than said predetermined change width.